This invention relates to graft polymer thermoplastic compositions made flame retardant by the incorporation of a phosphorus-substituted acrylonitrile monomer in the polymer. More particularly, the invention relates to graft polymers with improved flame retardant characteristics having incorporated therein an alpha or beta dialkoxy or alkyl alkoxy phosphinyl-substituted acrylonitrile as one of the grafting monomers.
Flame retardant properties have been imparted to thermoplastics by a variety of techniques. The most common method of the prior art is the compounding with flame retardant additives such as halogen compounds and organophosphorus compounds, and a great number of such additives are presently available for this purpose. In order to achieve adequate flame retardant character, large amounts of these additives are employed, and frequently greater than 30% of the total thermoplastic composition will consist of flame retardant additives. It is usually necessary to also include in the composition a synergist such as antimony oxide, zinc borate or the like to achieve maximum flame retardant properties. These synergists are known to enhance the activity of fire retardants, and may permit some reduction in additive level particularly when used in combination with the halogen-containing additives.
The use of high levels of flame retardant additives in thermoplastics suffers several disadvantages. Many of the additives are subject to loss from the composition during processing and subsequent use because of volatilization, leaching and/or bleeding. Flame retardant additives, and particularly those which are brominated materials, are often expensive and use of large amounts substantially increases the cost of the final composition. Many of the synergists are potentially toxic as well as expensive and in short supply, and a reduction in the use of these componds would be desirable both from an economic and an environmental standpoint.
The development of a useful thermoplastic material exhibiting good flame retardant properties alone or when compounded with a minimal level of a flame retardant additive, and which does not require use of synergistic compounds to achieve a high degree of flame retardant character would clearly be a useful advance in the art.